Not only were there more cases, but more cases were classified as severe in 2011 than in 2010 -- 275 versus 183, a difference that was significant at P=0.0018.

The CDC report suggested that the increased numbers might be a result of growing international travel to malarial regions, combined with inadequate chemoprevention by travellers.

"Malaria isn't something many doctors see frequently in the United States thanks to successful malaria elimination efforts in the 1940s," according to CDC Director Thomas Frieden, MD.

"The increase in malaria cases reminds us that Americans remain vulnerable and must be vigilant against diseases like malaria because our world is so interconnected by travel," he said in a statement.

The U.S. figures also appear to parallel increases in other countries, the agency said, noting that 2011 numbers in the United Kingdom were up 22% from 2008, although down slightly from 2010.

The findings are based on data submitted to the National Malaria Surveillance System, the National Notifiable Diseases Surveillance System, and the Armed Forces Health Surveillance Center.

Of the cases classified as imported, data on where the disease was acquired were available for 1,655, the agency reported, including 1,144 originating in Africa, 363 in Asia, 140 in the Americas, seven in Oceania, and one in the Middle East.

Although most of the imported malaria cases came from Africa, the single country that gave rise to the largest number of cases, some 223, was India. The next largest single-country source was Nigeria, with 213 cases.

The CDC report said that 871 patients reported the purpose of their travel, including 607 who said they were visiting friends or relatives.

Among the 929 cases in civilians with information on the use of chemoprophylaxis and region of travel, only 57 said they had adhered to a CDC-recommended chemoprophylactic regimen.

The five fatal cases included:

A 45-year-old Haitian immigrant who had returned from Haiti nine days before diagnosis after visiting friends and family in the Port-au-Prince and St. Louis areas. He developed acute respiratory distress syndrome and later multisystem organ failure.

An 83-year-old woman -- a U.S. resident of Indian ethnicity with a history of coronary artery disease, diabetes, and gastrointestinal bleeding -- who had been traveling in India for several months before her death. She was found unresponsive at home two days after her return to the U.S. and later died.

A 4-year-old girl who became ill in Uganda while visiting relatives. The disease was not diagnosed there, despite repeated tests. The child became unresponsive at a U.S. airport and was pronounced dead an hour later.

A 33-year-old missionary from Liberia. She and her husband had returned from Liberia about a month before diagnosis to visit family. At hospital, clinicians found she had malaria parasitemia and Salmonella bacteremia; she later developed multisystem organ failure.

A 58-year-old Indian national with a history of coronary artery disease and diabetes. The man was in the U.S. on vacation when he complained of fever and nonproductive cough. A day later, he went to a hospital where he was found to be anemic and thrombocytopenic with lactic acidosis, severe hypotension, and respiratory distress. He died hours later.